Antitrust laws aren't the obstacle, not unless Intel were the buyer. The cross-licensing agreement is the obstacle. It's structured heavily in Intel's favor. If AMD gets bought out, they lose the rights to Intel patented technology that they previously licensed. This includes elementary feature sets like SSE, SSE2, and AVX. Intel does not lose rights to any AMD patented technology, not unless Intel gets bought out, and nobody could digest Intel.
Any buyer would have to negotiate a new agreement with Intel, one that Intel does not have to negotiate, in order to avoid seeing 70 percent of AMD's revenue implode as soon as the deal goes through. What's left of the discrete CPU and APU business. The console chips and semi-custom designs using x86 cores. The K12. Gone.
AMD's CFO claims this is not so, but his word carries no ring of truth that I can see. The agreement is public, and others have studied it carefully. If AMD gets acquired and Intel doesn't, AMD and its successor are shut out of nearly $4 billion of revenue (annual, based on 2014).